1. Field of the Invention
The present invention relates to a near-infrared imaging sensor sensitive to near-infrared light.
2. Description of the Related Art
Infrared absorption bands of living organisms, foods, and the like are in the near-infrared range and therefore near-infrared imaging sensors are expected to be used for various applications in many surrounding fields. A known compound semiconductor which is sensitive to near-infrared light and which operates at room temperature is Indium gallium arsenide (InGaAs), which can detect light with a wavelength of up to 1.7 μm and is insensitive to light with a wavelength of more than 1.7 μm. Light with a wavelength of more than 1.7 μm can be detected with lead sulfide (PbS) and mercury cadmium telluride (HgCdTe, or MCT), which have low sensitivity and need to be cooled as disclosed in a non-patent document such as Masao Nakayama, “Sekigaisen Kenshutsusoshi no Gijutsu DoKo”, Sensor Technology, vol. 9, no. 3 (March 1989), pp. 61-64.
For actual devices, such as image pickup devices, using visible light, cost efficiency and reduction in size and weight are important factors. Many techniques, in which wavelength ranges are limited, have been proposed for reduction in size and weight. Some of the techniques are exemplified below.
(1) A solid-state image pickup device including a cylindrical housing containing an image pickup element such as a charge-coupled device (CCD) or a complementary metal-oxide semiconductor (CMOS) sensor and a lens (Japanese Unexamined Patent Application Publication No. 2002-247455).(2) A configuration in which a transparent cover has a hologram so as to have a simple structure and so as to be capable of color recognition (Japanese Unexamined Patent Application Publication No. 5-164922).(3) An image pickup device including a housing having a glass lid having a circular zone plate for reduction in size and weight (Japanese Unexamined Patent Application Publication No. 5-110959).
According to Techniques (1) to (3), a certain reduction in size and weight is available.
The above techniques have unsolved problems, which are as summarized below.
(1) and (2): (i) Since an image sensor used requires an optical lens, a filter, and the like, an image pickup system is complicated and has a large size. (ii) Since an increased number of components are necessary and high alignment accuracy is required to assemble a complicated structure, stable manufacture is difficult and yield is low.(3): The glass lid has an infrared absorption peak resulting from the vibration of hydroxyls in the 2-μm band. It is difficult to use glass to fabricate a micro-structure with high precision. Therefore, light fluctuates depending on the wavelength thereof and cannot pass through such a micro-structure.